1. Field of the Invention
This invention relates to a scanning optical apparatus and an image forming apparatus using the same, and is suitable for an image forming apparatus such as, for example, a laser beam printer (LBP), a digital copying machine or a multi-function printer having an electrophotographic printing process.
2. Description of the Prior Art
There have been proposed various scanning optical apparatuses for use in an image forming apparatus such as a laser beam printer, a digital copying machine or a multi-function printer having an electrophotographic printing process (see Japanese Patent Application Laid-open No. 2003-156704).
In such optical scanning apparatuses, a light beam (laser beam) emitted from light source means comprising, for example, a semiconductor laser or the like is converted into a parallel light beam by a collimator lens, and is directed to the deflecting and reflecting surface (deflecting surface) of a light deflector comprising a polygon mirror.
The light beam deflected by the light deflector is imaged into a spot shape on a surface to be scanned by an imaging optical system (fθ lens system), and the surface to be scanned is scanned at a constant speed with the light beam.
Also, in the scanning optical apparatus of this type, the parallel light beam emitted from the collimator lens is condensed on the deflecting and reflecting surface in a sub-scanning direction (in a sub-scanning cross section) orthogonal to a deflecting direction (main scanning direction) by a cylindrical lens.
Thereafter, an optical face tangle error correction optical system is used in which the light beam is re-imaged on the surface to be scanned by the imaging optical system.
In recent years, printing performance of a high printing speed and high definition has been required in the image forming apparatus such as the laser beam printer, the digital copying machine or the multi-function printer.
In any case, it is necessary to increase the frequency with which the surface to be scanned is scanned per unit time and therefore, this necessity has heretofore been coped with by increasing the number of surfaces of the polygon mirror or increasing the number of revolutions of the polygon mirror.
These methods, however, gives rise to a new problem that the polygon mirror becomes bulky and the load to a driving motor is increased to thereby produce a temperature rise, sound noise and the compactness of the apparatus is lost.
So, as methods of reducing the load to the light deflector, for example, various multi-beam scanning methods have been proposed in which the number of the light emitting points of a semiconductor laser which is light source means is increased so as to scan a surface to be scanned with a plurality of deflected light beams simultaneously.
The type of the light source of the multi-beam scanning method is divided broadly into two types.                A first type is a type in which a plurality of light source elements each emitting a single laser beam are arranged and a plurality of light beams are obtained by the use of optical path combining means such as a polarizing beam splitter or a half-mirror.        A second type is a so-called monolithic multi-beam type in which a plurality of light emitting points are constructed on a single light source element.        
The first type can use a single laser emitting element easy and simple (inexpensive) to manufacture, while on the other hand, it requires beam combining means and this leads to the problem that the entire apparatus becomes complicated and bulky.
In contrast, the monolithic multi-beam type, if a light source element can only be manufactured, requires no beam combining means and can make a scanning optical apparatus simple and compact.
The light source element of this monolithic multi-beam type is divided broadly into two types. They are:                a horizontal direction light emitting type; and        a vertical direction light emitting type.        
Any of these is manufactured by a semiconductor process, but they are classified by the emitting direction of the beam, a horizontal direction or a vertical direction to an element construction laminated on a wafer base surface.
In semiconductor lasers generally used at present, the horizontal direction light emitting type has become a mainstream because of the ease to manufacture. If a multi-beam light source is constituted by the horizontal direction light emitting type light source elements, the light beams are one-dimensionally arranged.
The horizontal direction light emitting type is sometimes called an edge emitter type.
In contrast, the vertical direction light emitting type light source element can emit a light beam vertically to the base surface thereof and therefore, light emitting points can be two-dimensionally arranged on the base surface, and this type is called a laser light source of a vertical cavity surface emitting type (hereinafter simply referred to as the “vertical cavity surface emitting laser”).
This vertical cavity surface emitting laser can easily increase the number of light emitting points by being two-dimensionally arranged, and has been particularly attracting attention in recent years.
Various scanning optical apparatuses using the surface light emitting layer of this vertical direction light emitting type (see Japanese Patent Application Laid-open No. 2002-040350 and Japanese Patent Application Laid-open No. 2005-011997) have been proposed.
On the other hand, an optical element such as an image lens used in the scanning optical apparatus is generally formed by molding by a mold. The molding by a mold has the merit that if a mold is once made, even a lens of a complicated shape can be simply manufactured.
Also, an aspherical shape is positively adopted into molded articles to thereby facilitate an improvement in optical performance and the curtailment of the number of lenses. Particularly it has been devised from old times to make the lens surface aspherical in a main scanning direction, whereby improvements in coma aberration and fθ characteristic in the main scanning direction have been achieved.
When the vertical cavity surface emitting laser as described above is used as the light source means of the scanning optical apparatus, various problems shown below arise. It is known that in the vertical cavity surface emitting laser, as disclosed in Japanese Patent Application Laid-open No. 2002-040350 and Japanese Patent Application Laid-open No. 2005-011997, if a driving current increases, the emission angle of a light beam varies.
The emission angle of a laser beam can be obtained by evaluating the far field pattern (FFP) of a laser beam emitted from a laser light source with respect to the emission angle, where the far field pattern (FFP) is indicated by being normalized by the intensity of a light beam in the normal direction (angle 0°) of a laser element as shown in FIG. 10.
Here, a half value angle of the far field pattern refers to a beam pattern at a point far by several tens of millimeters or more from a laser emitting port.
As is generally known, the distribution of the far field pattern of a laser beam assumes a Gaussian distribution with respective to the emission angle.
Generally, an index representative of the expanse of a light beam is indicated by the difference between two angles at which the emitted beam intensity having the emission angle dependency becomes a half value of a peak value, and it is sometimes called FWHM (half value width). Herein, it is expressed as “the half value angle of the far field pattern”.
The half value angle of the far field pattern of a vertical cavity surface emitting laser is narrower than that of the edge emitter laser, and generally is of the order of 10°–15°. Also, the difference between the half value angles of far field patterns in two planes containing the normal and orthogonal to each other is very small as compared with that of the edge emitter laser.
This is attributable to the fact that in the vertical cavity surface emitting laser, the diameter of a light emitting area is made small (generally several microns to 20 microns) for the stabilization of an oscillation mode and also, a light emitting portion laminated in a vertical direction on a laser substrate is constructed generally rotation-symmetrically.
On the other hand, it is also known that if the diameter of the light emitting area is made too small, the light emission amount becomes too small, and there is also a limit to making the diameter small.
In the vertical cavity surface emitting laser, if a drive current is increased (automatic power control: APC) to increase a light amount output or to compensate a reduction in light emitting efficiency due to a change in temperature of a substrate, the oscillation mode becomes unstable and mode distribution noise is liable to be caused. As the result, the emission angle of the light beam fluctuates as described above.
Depending on the structure of the laser element, in a popular vertical cavity surface emitting laser, the expanse angle of the light beam widens as the drive current increases, that is, a half angle value (FFP) of the far field pattern increases.
Also, it is known that the amount of the change is great in a direction and the amount of the change is small in another direction, as disclosed in FIG. 10 of Japanese Patent Application Laid-open No. 2005-011997. The angles 12°, 18° and 24° shown in the figure are the difference between two angles at which the beam intensity of the emission angle dependency of the emitted beam intensity becomes a half value of the peak value, as described above. Specifically, when the light intensity is viewed at a slice of 0.5, the emission angles are ±6°, ±9° and ±12°, respectively, and the differences are 12°, 18° and 24°, respectively.
In such a scanning optical system as disclosed in Japanese Patent Application Laid-open No. 2003-156704, a stop is provided in an optical path to thereby limit a light beam emitted from a laser light source and adjust the shape of the light beam to a particular shape.
Also, as described above, the cross section of the laser beam assumes a Gaussian distribution and therefore, on the pupil in the imaging optical system wherein the stop is imaged, the light intensity distribution assumes different intensities between the center of the pupil (pupil center) and the peripheral portion of the pupil (pupil edge portion).
If the emission angle of the light beam emitted from the laser light source=the expanse of the Gaussian distribution=FFP half value angle fluctuates, the light intensity distribution on the pupil also fluctuates. If for example, FFP half value angle widens, the light intensity distribution on the pupil also widens, and the light intensity ratio at the pupil end portions relative to the pupil center as the index of the light intensity distribution on the pupil also increases.
Consequently, the profile of an imaged spot on the image plane which can be calculated as the FFP of the pupil intensity distribution also fluctuates.
FIGS. 11 and 12 show how the imaged spot on the image plane is changed depending on the light intensity distribution on the pupil of the imaging optical system. FIG. 11 shows a change in the spot diameter on the best image plane def=0 and defocus def=3 mm, and the axis of abscissas indicates the light intensity ratio at the pupil edge portion to the pupil center as the index of the light intensity distribution on the pupil, and the axis of ordinates indicates 1/e2 (e being the base of natural logarithm) to the peak intensity as the spot diameter of the imaged spot. Also, FIG. 12 shows an image plane defocus change in the spot diameter, and the axis of abscissas indicates the defocus amount in the in-focus direction, and the axis of ordinates indicates 1/e2 to the peak intensity as the spot diameter of the imaged spot, and shows the light intensity ratio at the pupil edge portion relative to the pupil center as it is changed to 0.1–1.0.
As can be seen from FIGS. 11 and 12, the lower the ratio of the light intensity of the pupil edge portion becomes, the larger the spot diameter becomes, and conversely, the more the ratio of the light intensity of the pupil edge portion is increased, the smaller the spot diameter becomes.
That is, it can be seen that the FFP half value angle of the vertical cavity surface emitting laser shown in FIG. 10 has been fluctuated depending on the drive current as described above, and the ratio of the light intensity of the pupil edge portion has been fluctuated, whereby the spot diameter is also fluctuated.
If for example, the drive current is increased in an attempt to increase the light emission amount, the half value angle of the far field pattern widens and the ratio of the light intensity of the pupil edge portion becomes great, and the spot diameter becomes small.
It is known that in the electrophotographic printing process, a latent image is formed on the surface of a photosensitive member (photosensitive drum) by a laser spot, and the latent image is developed as a toner image by a known electrophotographic process, and is transferred to a recording medium (paper).
As is apparent from this, if a spot diameter forming the latent image is fluctuated, the size of the latent image is also changed.
For example, in a thin line as an image obtained by the dots of a laser spot being formed in a row, or a dot matrix as an image obtained by the dots of a laser spot being formed at a constant period, if the size of the laser spot changes, the line width of the thin line or the dot size of the dot matrix is changed.
In an image forming apparatus for a black-and-white image, the line width of the thin line or the dot size of the dot matrix is recognized as difference in density by an examinee.
Also, in an image forming apparatus for a color image, the line width of the thin line or the dot size of the dot matrix is recognized as a color difference by the examinee.
As described above, the stability of an image is aggravated by a change in the spot diameter.